A photodiode is a type of photo-detector capable of converting light into current or voltage. Generally, the photodiode is a semiconductor device with a PIN or PN connection structure. When a photon of sufficient energy strikes the photodiode, it excites electrons, creating free electrons and positively charged electron holes. The holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced proportional to the amount of incident light on the photodiode.
Other types of sensors include e.g. pyro-electric, piezo-electric, or capacitive sensors.
All of these sensors are common in that they include a pair of electrodes and, when stimulated by their respective stimuli, the state of charge across the electrodes changes. Current resulting from the changed state of charge across the electrodes can then be used to detect and quantify the stimuli. For example, a photodiode produces a change in the state of charge across its electrodes when the light is incident on the photodiode. In other words, the photodiode generates current (which may be referred to as “photocurrent”) in the presence of light, where the current is proportional to the amount of light incident on the photodiode. Similarly, a pyro-electric sensor produces a change in the state of charge across its electrodes when heated or cooled, a piezo-electric sensor produces a change in the state of charge across its electrodes in response to the change in its mechanical orientation (e.g. strain), while a capacitive sensor is one where changes in the environment change the effective capacitance of the sense element, which in turn changes the capacity to hold charge.
For such sensors, as well as other sensors operating according to similar principles of detecting change of charge state, it may be desirable to be able to detect and quantify stimuli originating from a specific source of interest as well as stimuli originating from all other sources besides the source of interest. For example, in context of a photodiode, it may be desirable to detect and quantify a contribution to the current generated by the photodiode that is due to the detection of light generated by a particular light source of interest, e.g. a particular light emitting diode (LED), as well as to detect and quantify contribution to the photocurrent that is due to the detection of light generated by all other light sources besides this light source of interest.